1. Field of the Invention
The present invention relates to a circuit board, which is mounted to electric/electronic apparatuses adapting to a high frequency current.
2. Description of the Prior Art
Conventionally, a circuit board formed in the following manner has been known. More specifically, a conductive circuit is formed on a base material (insulating substrate) made of synthetic resin excellent in a dielectric strength. In the circuit board thus formed, the conductive circuit is formed using copper as a base excellent in a conductivity (electric conductivity). However, the copper is inferior to a corrosion resistance; for this reason, a material having a high conductivity and excellent corrosion resistance must be laminated on a layer made of cooper. Preferably, gold is usable as the above material. However, the gold is diffused in copper; for this reason, in general, a slight thick anchor layer of nickel (about 4 to 5 xcexcm) is laminated on the surface of copper, and further, gold is laminated on the anchor layer. Thus, a circuit board 11 as shown in FIG. 5 has been known as a conventionally normal circuit board. The conventional circuit board is formed in the following manner. More specifically, a layer 12 made of copper is laminated on a base material 16, and a nickel plating layer 14 is laminated on the layer 12 made of copper, and further, a gold plating layer 15 is laminated on the nickel plating layer 14. In this case, gold is high expensive; for this reason, normally, the gold is plated extremely thin, that is, less than a thickness of about 0.1 xcexcm.
In general, in the case where a high frequency current flows through a conductor, a skin effect is generated such that the current is limited to a surface layer of circuit; for this reason, almost no current flows through the conductor. Therefore, for example, in the case where the aforesaid circuit board 11 is used for electric/electronic apparatuses adapting to high frequency current, such as a high performance amplifier and converter, a current having a very high frequency of about 20 GHz flows through the circuit. For this reason, theoretically, a current flows through a part up to a depth of about 0.45 xcexcm from the surface of circuit. Thus, in the conventional circuit board 11, most of current flows through the layer 14 made of nickel; for this reason, a conduction loss becomes high. As a result, a problem arises such that a noise figure of the used high performance amplifier and converter increases.
The present invention has been made in order to solve the above problem arisen in the conventional circuit board such as the above circuit board 11. It is, therefore, an object of the present invention to provide a circuit board, which can reduce a conduction loss and generate no trouble in electric/electronic apparatuses in the case of being mounted to electric/electronic apparatuses adapting a high frequency current.
Inventors of the present invention have found the following matter. More specifically, in a circuit part of circuit board, a material having a high conductivity is much provided on a part up to a depth of about 0.45 xcexcm from the surface of circuit through which a frequency current of about 20 GHz flows, and thereby, it is possible to reduce a conduction loss of the circuit board for high frequency, and thus, the inventors have proposed the present invention. According to a first aspect, the present invention provides a circuit board, which is constructed in a manner that a conductive circuit is formed on a base material made of polytetrafluoroethylene (Teflon etc., manufactured by du Pont) the conductive circuit being formed by laminating a layer made of tin on a layer made of copper.
Further, according to a second aspect, the present invention provides the circuit board wherein the layer made of copper has a thickness less than 0.45 xcexcm.
Furthermore, according to a third aspect, the present invention provides a manufacturing method of a circuit board, comprising the following steps of: forming a copper foil on a base material made of polytetrafluoroethylene; subjecting copper plating onto the copper foil layer; and subjecting tin plating onto the copper plated layer.